Classical class I molecules present endogenous peptide antigens for recognition by cytotoxic T lymphocytes, which are transported by a heterodimer encoded by the TAP1 and TAP2 genes. Nonclassical class I (class Ib) molecules, although similar in structure to classical class I molecules, have an unknown function. We are interested in studying the cell biology and biochemistry of the representative mouse class Ib molecules, Q7b and mCD1.1, with the ultimate goal of determining whether nonclassical class I molecules can present viral antigens to cytotoxic T cells. To study these proteins in a variety of tissue culture cell lines in a strictly controlled manner in vitro and in mice we produced vaccinia virus recombinants expressing these proteins. Using these recombinants we made the following observations. 1. Cell surface expression of both Q7b and mCD1.1 requires assembly with b2-microglobulin. 2. Q7b expression also requires cells to express TAP, while mCD1.1 expression is TAP independent. 3. Consistent with its functional association with TAP, GPI anchored Q7b physically associates with TAP. 4. The antigen presentation function of mCD1.1 to a unique subset of T-cells (CD4-CD8- NK1.1+) parallels the requirement for surface expression.